Referring to FIGS. 1 and 2, U.S. Pat. No. 6,675,861 discloses a brake for a cordless blind in which a window covering 1 includes a head rail 11, a bottom rail 12 and a blind 13 located between the head rail 11 and the bottom rail 12. The window covering 1 also has a lift cord assembly 14 linking to the blind 13. The bottom rail 12 has a lifting control apparatus 15 which has a button 151 extended from the front side of the bottom rail 12 and a retaining member 152 held in the bottom rail 12 in an extensible manner through a spring 153. The button 151 and the retaining member 152 can clamp the lift cord assembly 14 to harness the movement of the lift cord assembly 14 and the blind 13. Referring to FIG. 3, when in use to retract the blind 13 upwards or extend it downwards, depress the button 151 to compress the retaining member 152 through the spring 153, the lift cord assembly 14 can be moved smoothly between the button 151 and the retaining member 152 so that the blind 13 can be retracted upwards or extended downwards. The window covering 1 can be installed on a door or window of a house to block sunshine and provide decoration purpose.
However in practice the lifting control apparatus 15 mentioned above still has problems, notably:
1. Poor locating: The lifting control apparatus 15 clamps the lift cord assembly 14 through merely the button 151 and retaining member 152. As the general lift cord assembly 14 has to bear the weight of the blind 13, when the weight of the blind 13 is great, the locating effect of the lifting control apparatus 15 on the lift cord assembly 14 is limited. As a result, retracting or extending the window covering 1 is not always easy or smooth.
2. Shorter life span: The function of the lifting control apparatus 15 is mainly achieved through the elastic expansion and compression of the spring 153 to clamp the lift cord assembly 14 between the button 151 and the retaining member 152. As the lift cord assembly 14 has to bear the weight of the blind 13, the spring 153 is subject to a great force. After used for a period of time, elastic fatigue occurs to the spring, and the clamping and locating effect of the button 151 and retaining member 152 diminish. The life span of the brake means 15 suffers.
Referring to FIG. 4, U.S. Pat. No. 6,029,734 discloses a slat-lifting mechanism for Venetian blinds in which a lifting control apparatus 15a also provides an elastic extensible function through a spring 153a so that two pull cords 14a and 14b can be clamped between a button 151a and a retaining member 152a. As the two pull cords 14a and 14b have to bear the weigh of the blind, the single spring 153a also receives a greater force. And elastic fatigue also occurs after used for a period of time. As a result the clamping and locating effect of the button 151a and retaining member 152a diminish. And the life span of the lifting control apparatus 15a also shortens.